1. Field of the Invention
The present invention relates to an active matrix type display device having an SRAM in a pixel. More specifically, the present invention relates to a circuit technology reducing a power loss caused during display of a still image by graphic data held in the SRAM.
2. Description of the Related Art
As a memory device capable of statically holding graphic data in one pixel, an active matrix type liquid crystal display device having an SRAM built therein (hereinafter, referred to as an SRAM-built-in liquid crystal display device) is developed.
Generally, in a liquid crystal display device without the SRAM built therein, still image data is given to each frame, thus displaying a still image. Since a driver, a graphic controller and the like continuously operate during the display, it is difficult to reduce power consumption. Meanwhile, in the SRAM-built-in liquid crystal display device, a still image is displayed based on the still image data held in the SRAM (hereinafter, referred to as SRAM holding data). Since the driver, the graphic controller and the like are on standby during the display, the power consumption can be reduced. As documentation disclosing this type of liquid crystal display device, there is U.S. Pat. No. 5,712,652. Here, described is a liquid crystal display device including a digital memory cell as a memory device for each pixel.
During the display of the still image based on the SRAM holding data, it is unnecessary to supply a power source voltage to circuits of the driver, the graphic controller and the like on standby. In the conventional SRAM-built-in liquid crystal display device, since the power source voltage has been supplied to the entire circuits even on standby, the power loss has occurred inside the circuits on standby.
A similar power loss occurs also in a DC/DC converter supplying the power source voltage to the driver, the graphic controller and the like. The DC/DC converter is constituted of a switching regulator or a series regulator. Therefore, even if a load thereto is almost zero, a self loss of such a regulator is caused, leading to a power loss for this amount.
In many cases, the SRAM-built-in liquid crystal display device is used as a display of a portable information apparatus driven by a battery. Hence, a wasteful power loss causes a battery life to be shortened. From the background as described above, for the SRAM-built-in liquid crystal display device, required is further reduction of the power consumption during the display of the still image based on the SRAM holding data.